This invention relates to a vehicle drive system. In particular, this invention relates to a system provided with an automatic transmission which has a plurality of gear positions and which varies a speed ratio in a step manner.
JP-A-H11-78620 published by the Japanese Patent Office in 1999 discloses a vehicle drive system. This system is used in a vehicle mounting a continuously variable transmission (CVT). A target engine torque is calculated by dividing a target drive force by the actual speed ratio (ratio of the output rotation speed to the input rotation speed of the transmission). In this manner, an optimal target engine torque is produced which constantly corresponds to the speed ratio without the need for a complicated control routine. Furthermore the step change of the drive force before and after the speed change is suppressed.
Since a CVT performs continuous and smooth variation of a speed ratio, the target engine torque does not undergo sharp variation even when calculated in the above manner.
However when the drive force control is performed in a vehicle provided with an automatic transmission in which sharp variation in the speed ratio occurs over a short period of time, when the target engine torque is calculated by dividing the target drive force by the actual speed ratio in the above manner, the target engine torque, that is to say, the input torque to the transmission undergoes sharp variation during speed change.
If it were possible to make the timing of variation in the actual speed ratio completely coincide with the timing of variation in target engine torque, it would be theoretically possible to prevent the generation of shift shocks by making the realized drive force equal the target drive force. However in practice, shift shocks result from a degree of deviation between the respective timings. Furthermore attempts to align the respective timings require minute regulation of the speed change timing including regulation of the hydraulic control system of the transmission. In particular, since the target engine torque undergoes sharp variation for the reasons outlined above, considerable difficulty is encountered in varying the speed ratio with respect to the target engine torque.
It has been proposed to solve the above problems by generating a target engine torque during speed change without reference to the actual speed ratio. However although this arrangement is effective in suppressing shift shocks, the resulting drive force does not obtain the desired target drive force due to fluctuations in speed change time. Therefore the drive force fluctuates before and after speed change and a differential results in the drive force.
It is therefore an object of this invention to prevent the generation of shift shocks by suppressing sharp variation in the engine torque during speed change when controlling the drive force of a vehicle provided with an automatic transmission which varies a speed ratio in a step manner.
In order to achieve above object, this invention provides a vehicle drive system comprising an engine, an automatic transmission connected to the engine, the transmission provided with a plurality of gear positions and varying a speed ratio in a step manner, a sensor which detects a running condition of the vehicle and a controller. The controller functions to calculate a target drive force based on the running condition of the vehicle, calculate a delay speed ratio which varies with a delay on an actual speed ratio of the transmission, calculate a target engine torque by dividing the target drive force by the actual speed ratio during steady-state conditions, and calculate the target engine torque by dividing the target drive force by the delay speed ratio while the actual speed ratio is undergoing variation and control the engine torque so that the engine torque coincides with the target engine torque.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.